This invention relates to a circuit arrangement for feeding a lamp comprising
a first input terminal K1 and a second input terminal K2 which are to be connected to a supply voltage source supplying a DC voltage,
an inverter for generating a square-wave periodic voltage from said DC voltage, which inverter is provided with a series arrangement of a first switching element S1, a first inductive element L1, a second inductive element L2 and a second switching element S2, and which inverter interconnects the input terminals,
a control circuit which is coupled to a control electrode of the first switching element S1 and to a control electrode of the second switching element S2, which control circuit is used to generate a control signal for rendering the first and the second switching element alternately conducting and non-conducting,
a load branch comprising a third inductive element L3, lamp terminals for connecting the lamp, and a first capacitive element C1,
a first unidirectional element D1 having an anode coupled to the second input terminal K2 and a cathode coupled to a point between the first switching element S1 and the first inductive element L1,
a second unidirectional element D2 having a cathode coupled to the first input terminal K1 and an anode coupled to a point between the second switching element S2 and the second inductive element L2.
Such a circuit arrangement is disclosed in WO-9902020. In the known circuit arrangement, the control circuit is also provided with a dimmer circuit for dimming the lamp by regulating the duty cycle of the control signal. In addition, the self-inductances L1xe2x80x2, L2xe2x80x2 and L3xe2x80x2 of, respectively, the first, the second and the third inductive element L1, L2 and L3 are chosen so as to be substantially equal to each other. The first and the second inductive element are magnetically coupled to each other and hence jointly form a transformer. As a result of said values of the self-inductances and by virtue of this magnetic coupling, it is achieved that the shape of the current through the lamp during dimming the lamp comes fairly close to a sine shape. In other words, the lamp current comprises comparatively few higher harmonic terms, as a result of which the amount of disturbance generated by the lamp is limited. In addition, in the known circuit arrangement, acoustic resonances are effectively suppressed. In a part of the range wherein the duty cycle of the control signal can be regulated xe2x80x9chard switchingxe2x80x9d occurs. This means that each one of the switching elements is rendered conducting while a comparatively high voltage is present across the switching element. This may give rise to a comparatively high power dissipation in the switching elements. In the known circuit arrangement, this power dissipation is counteracted to a limited extent only as a result of the fact that the first and the second inductive element are arranged in series with the switching elements. In addition, a drawback of the known circuit arrangement resides in that the transformer formed by the first and the second inductive element is a comparatively expensive and bulky component.
It is an object of the invention to provide a circuit arrangement wherein the power dissipation caused by xe2x80x9chard switchingxe2x80x9d is effectively counteracted using comparatively straightforward, inexpensive and small components.
To achieve this object, a circuit arrangement as mentioned in the opening paragraph is characterized, in accordance with the invention, in that with respect to the self-inductances L1xe2x80x2, L2xe2x80x2 and L3xe2x80x2 of, respectively, the first, second and third inductive element, the following relationship applies;
L3xe2x80x2 greater than 5*L1xe2x80x2 and L3xe2x80x2 greater than 5*L2xe2x80x2.
In a circuit arrangement in accordance with the invention, power dissipation in the switching elements due to xe2x80x9chard switchingxe2x80x9d is substantially suppressed in spite of the comparatively small self-inductances of the first and the second inductive element. Power that would be dissipated in the switching elements, if the first and the second inductive element and the first and the second unidirectional element were absent, is effectively fed back to the supply voltage source or used to generate a current through the lamp. It has been found that this applies if the first and the second inductive element are magnetically coupled, but also if the inductive elements are not coupled.
It has been found that in many cases power dissipation is very effectively counteracted if with respect to the self-inductances L1xe2x80x2, L2xe2x80x2 and L3xe2x80x2 of, respectively, the first, second and third inductive element, it applies that
xe2x80x83L3xe2x80x2 greater than 10*L1xe2x80x2 and L3xe2x80x2 greater than 10*L2xe2x80x2.
It has also been found that power dissipation can be further reduced if the circuit arrangement is additionally provided with a third unidirectional element D3 and a fourth unidirectional element D4, with a cathode of the third unidirectional element D3 being coupled to the first input terminal K1, an anode of the fourth unidirectional element D4 being coupled to the second input terminal K2 and an anode of the third unidirectional element D3 and a cathode of the fourth unidirectional element D4 each being coupled to a point between the first inductive element L1 and the second inductive element L2.
As the circuit arrangement comprises parasitic capacitances, oscillations occur which are brought about by the first and the second inductive element and said parasitic capacitances. By means of the third and the fourth unidirectional element it is achieved that the amplitude of voltages caused by these oscillations, particularly of the voltage on the point between the first and the second inductive element, remains limited. A further reduction of the power dissipation is thus achieved. In addition, the unidirectional elements D3 and D4 form part of current paths for xe2x80x9creversexe2x80x9d currents having a small impedance. As a result, in the case of xe2x80x9chard switchingxe2x80x9d, the third unidirectional element D3 carries current, not the second unidirectional element D2, for rendering the second switching element S2 conducting. Correspondingly, the fourth unidirectional element D4 carries current, not the first unidirectional element D1, for rendering the first switching element S1 conducting. By virtue thereof, power dissipation in the first and the second unidirectional element and the switching elements is limited substantially when the switching elements are becoming conducting.
Field effect transistors such as MOSFETs are often used as the switching elements in a circuit arrangement in accordance with the invention. Such field effect transistors comprise an internal diode that is capable of guiding the current in a direction that is in opposition to the direction in which the field effect transistor carries current in the conducting state. These internal diodes play an important part in the functioning of the circuit arrangement since they carry current during specific operational phases of the circuit arrangement. If these internal diodes are comparatively slow, then a comparatively high power dissipation occurs when said internal diodes become non-conducting. This contribution to the power dissipation can be reduced substantially if the circuit arrangement is additionally provided with a fifth unidirectional element D5 which is arranged in series with the first switching element S1, a sixth unidirectional element D6 which is arranged in series with the second switching element S2, a first shunt branch which comprises a seventh unidirectional element D7 and shunts the series arrangement of the fifth unidirectional element D5 and the first switching element S1, and a second shunt branch which comprises an eighth unidirectional element D8 and shunts the series arrangement of the sixth unidirectional element D6 and the second switching element S2. Said unidirectional elements D5-D8 being chosen so as to operate at a comparatively high speed with respect to the internal diodes of the switching elements S1 and S2.
As indicated hereinabove, xe2x80x9chard switchingxe2x80x9d occurs particularly in a circuit arrangement wherein the control circuit is provided with a dimmer circuit for regulating the duty cycle of the control signal. Consequently, the invention can very advantageously be used in such circuit arrangements.
Controlling the luminous flux of the lamp by means of a dimmer circuit for regulating the duty cycle of the control signal can be very advantageously applied in circuit arrangements which are intended to feed lamps of a different type, since the relation between the duty cycle of the control signal and the luminous flux of the lamp is very similar for lamps of a different type. Such circuit arrangements intended to feed lamps of different types are generally provided with a circuit part for recognizing the type of lamp connected to the lamp terminals.